Geochemistry of Uranium and Rare Earth Elements in the Phosphate Horizon of Jeiroud Formation, Firoozkuh area, Central Alborz Zone

Two phosphorite deposits including Paghela and Gadok, within the Jiroud Formation (Upper Devonian) in the northeast of Firoozkuh in the Central Alborz geotectonic zone have been studied. Microscope studies show that fluorapatite is the main phosphate mineral and the main gangue minerals are calcite and quartz. The P2O5, Uranium and ∑REE contents in the Paghaleh phosphorite range between 35.5-41.2%, 4.8-6.4 and 623-803.2 ppm, and in the Gaduk phosphorite vary from 12.3 to 41.1%, 4.7 to 7.8 and 369.2 to 784.8 ppm, respectively. The REE patterns normalized to Post-Archean Australian shales in Firoozkuh phosphorites are characterized by convex patterns without specific Ce anomaly, which indicates anoxic conditions in the diagenesis environment of the phosphorite formation. Mineralogical and geochemical studies and the positive correlation of REEs with P2O5 and CaO show that the main host mineral of rare earth elements in the phosphorites of Firoozkuh is fluorapatite. Also, minor amount of zircon and trace amount of monazite inclusions in the apatite are other minerals that host rare earth elements in these phosphorites. The lack of uranium enrichment in Firoozkuh phosphorites is probably either due to the lack of favorable source rock with leachable uranium in the continent, or by the initial weak oxidant conditions during phosphatization, which prevented the conversion of U+6 dissolved in seawater to U+4, and uranium could not replace Ca within the apatite lattice.